Variable mu amplifier circuit



Patented Dec. Q, 1936 Stuart Ballantine, Mount il? LIFIEB ain Lakes, N. J., assigner to Radio Corporation of America, a corporation oi' Delaware Application July i8, 1931, Serial No. 551,625 i2 claim. (or. riei'zr) My present invention relates to methods of. and apparatus for, improving the operating characteristics of amplifiers of the electronic relay type.

In radio broadcast receivers the control of audio output for varying antenna signal voltages is commonly effected by varying an electrode voltage, such as the control grid bias o'f the radio amplifier tubes. With ordinary triodes as heretofore employed, the range of antenna radio frequency voltage over which the satisfactory control by this means can be obtained is limited, the results being unsatisfactory when the receiver is so located that the signal voltage from neighboring transmitting stations is high. In these cases several undesirable effects accompany the increasing negative control-grid bias which is necessary to secure constant audio output with increasing signal voltage. These eieets may be stated as follows:

(1) Distortion due to non-linearity in the relation between the radio frequency output voltage of the amplifier and the radio frequency input voltage;

(2) Increased inter-modulation or cross-talk eects.

It can be demonstrated that the underlying cause of these eects is the increase in the higher order curvature of the tube characteristic in relation to the eii'ects of the first order, which takes place if the applied radio frequency voltages are increased. l

The present invention has as its main object the reduction of the dimculties enumerated above with a minimum of inconvenience and readjustmente This has been achieved by so designing the effective plate current-grid voltage characteristlc as to minimize the higher order curvature over an extended range of control grid voltage. Before discussing the specific nature of the invention, and the principles of its operation there will be considered briefly the' underlying problems of distortion and cross-talk which it is intended to solve.

The problem of distortion considered herein is the special one associated with the control of amplification in the radio amplifier by variation of an electrode voltage, usually the control-grid bias. Important undesirable eiects which arise in an ampliiler so controlled are well known. It has been demonstrated in actual tests with an amplier that for values of grid bias below a certain value (as for example nine volts) the relation between output and input is linear up to a certain point. Beyond this point the curve is bent over by overloading due to grid current in the iinal tube. Above the aforementioned value, however, the output-input relations become non-linear, and an attempt to operate over this range of input voltage will result in a distorted output. This distortion is most severe for high degrees of modulation. It can, also, be shown that, for small degrees of modulation, the degree of modulation is effectively increased. For example, a signal of a certain amplitude having a modulation of ve per cent will be found to have its modulation increased to about seventy per cent in passing through the amplifier. This vrise in modulation may be referred to as modulation rise, andl is productive of the distortion sought to be eliminated by the present invention.

Considering, now, the problem of cross-talk, lt is pointed out that the term cross-talk is ernployed herein and after to designate that species of interference which originates in the radio frequency amplifier tubes by modulation between two, or more, signals. These modulation effects depend upon the higher-order curvature parameters of the tube, and are te that extent related to the problem of distortion which has just been discussed. It is to be expected, therefore, that characteristics and applied voltage amplitudes which conduce to distortion will be commensurably troublesome from the viewpoint of crosstalk, and any improvement which can be effected in an amplifier to reduce distortion will also reduce a considerable part of the crosstali:.

It is desired to particularly point out that the type ofcross-talk especially sought to be eiiminated is heard in circumstances of the oilowng type. The receiver is tuned to a signal, which may be called the desired signal. A second signal. which may be termed the extra signal, of different frequency,is also present at the grid of the rst tube. Assume, also, that the selectivity of the receiver and the frequency diierence between the signals are such as to exclude the possibility of the extra signal being heard by simple interference in the absence of desired signal. If the desired signal be modulated both signals are heard simultaneously, if unmodulated the extra signal is heard. It can be theoretically demonstrated that this type of cross-talk depends mainly upon the third-order derivative of the tube characteristics, that is, upon the same derivative that causes the distortion eect heretofore discussed.

Accordingly, it may be stated that it is an important object of the present invention to provide, in current amplifiers of the type in which the gain is controlled byadjustment o the control grid bias potential, novel methods of, and apparatus for. obtaining substantially distortionless ampliilcationand highly reduce the cross-talk when the transconductance is reduced by increasing the grid bias.

Still another important object of the present invention is to provide a method of reducing distortion and cross-talk in radio frequency ampliilers, particularly adapted for use in radio receivers, by means of an arrangement adapted to minimize the higher order curvature, particularly the points where the transconductanc is low.

A further object, generically explained, is to provide alternating current repeaters in which the higher order curvature of the plate currentgrid biascharacteristic is reduced by arranging, in parallel, space current paths having substantially diiferent mu-factors.

Still other objects of the present invention are to improve generally the emciency of radio frequency amplifiers, andv to particularly provide a radio frequency amplifier, especially adapted for use in connection with radio receivers, which is not only reliable in operation, but capable of substantially minimizing distortion andcrosstalk eiiects.

The novel features which I believe to be characteristic oi my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically one circuit arrangement whereby my invention may be carried into effect. f.

,In the drawing,

Fig. 1 is a circuit diagram of onel embodiment of the invention, v

Fig. 2 is a graphic illustration of the effective plate current-grid bias characteristic of the ampliiler shown in Fig. 1, and

Figs. 3a and 3b are graphical illustrationsof the underlying principle of the present invention.

To understand the principles underlying the present invention, reference is first made to Figs. 3a and 3b showing respectively decreasing transconductance. and increasing plate current accompanying attempts to decrease distortion by decreasing the mu-factor. As stated above. the solution of the problems of distortion and crosstalk in electron discharge tubes employed as amplifiers consists essentially in designing the effective plate`l current-grid bias characteristic of an amplifier in such a way as to minimize the higher order curvature, particularly at points where the transconductance is low.

Fig. 3a shows the grid-plate transfer characteristic A'l of an ordinary tube well known in the prior art. To raise the input voltage at which listortion appears the design could be changed.`

so as to decrease the mu-factor of the tube, thus spreading out the range of grid voltages, and obtaining a characteristic of the type shown by the curve A. This, however, entails a sacrifice in transconductance, and amplification, at the initial bias if the plate current at this point is adjusted to the same value. If the transconductance atthe initial bias is to be maintained at the same value instead. then anincrease in plate current must be tolerated as shown by the characteristic B in Fig. 3b.

A way out of this difficulty is illustrated in Fig. 1. The amplifier stage shown in Fig. 1 comprises grammatically, is adjustable for two space discharge tubes in parallel. Of these tubes, the reference numeral I designates a tube of the high mu type. while the reference numeral 2 denotes a tube of the low mu type.

The source of alternating current waves to be transmitted, which source may include avsignal selector device, as a tunable oscillation circuit, is connected across the input terminals I, l of the repeater. The grid circuits of the tubes include sources "l, l of bias voltage which are individual to the two tubes, and a common source l of grid bias voltage which, as indicated diacontrolling the gain of the repeter. 'I'he anode circuits of the tubes l, 2 have individual sources thefpurpose of 1, 8, respectivelyI of anode potential supply, and

The high-mu tube i nu i plats current-gris bias characteristic suchas shown by the dotted line A, and will give a high amplincation of weak signals when operated at relatively low negative bias potentials. 'Ihe characteristic of the low-mu tube is shown by curve B. and it is apparent that this tube can` handle highinput voltages but that its maximum amplification is low. The combination of the low and high mugtubes gives an operating characteristic such as shown by the solid line C.

In combination, the two tubes complement each other toy produce a characteristic of the desired type. For relatively small voltages applied at the input terminals 3, the bias potentials are adjusted, by appropriate control of the bias potential l, to relatively low values. andhigh amplification is obtained. Both of the tubes are active, but due to the higher transconductance of tube 2 most'of the amplincation is obtained in that-tube. As the bias increases negatively, the tube 2 is automatically cut oi'f. and the operation is gradually shifted to tube i which is capable of handling the increasing input voltages. In other words, the mu factor of the amplier decreases continuously, as the grid bias increases negatively to keep step with the increasingl input voltage.v

It will be apparent that the invention is not limited to the circuit arrangement shown vin Fig. l, nor to the production of an ampliiler having a plate current-grid bias characteristic such as shown in Fig. 2. By appropriate choice of the number and the characteristics of the individual space current paths that are utilized in the repeater, any desired relationship may be obtained between the grid bias potential and the mu factor of the repeater. The curvature of the plate current-grid bias characteristic, may of course, be further reduced by adding, in parallel with tubes l and 2, another-tube which has a mu factor intermediate those of the high and low mu tubes. l

While I have indicated and described one arrangement for carrying my invention into edect, it will be apparent to one skilled in the art that my invention'is by no means limited to the particular organize-tion shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

aocaaea What I claim is:

1. In a radio frequency ampliier, means establishing a space current path having in parallel a high mu section and a low musection, control electrode means in both sections of said path,

means for impressing a desired radio frequency signal upon said control electrode means, and means for adjusting the bias potential upon said control electrode means between a value effective to establish space current iiow in botliI of said sections of the space current path and a value eifective to establish a current flow 'in only one section of said space current path without modulation distortion of said signal or production of cross-talk eiects between said signal and undesired strong signals.

2. In a radio frequency amplifier. cathode and anode means between which a iiow of space current may be established, control means extending transversely of the path of space current flow, an input circuit connected between said cathode means and said control means for impressing desired radio frequency signals ou said amplifier, an output circuit for said space current path, said control means exercising substantially different degrees of control over current iiow in diierent portions of said path 'at diierent bias potentials, a source of direct voltage for impressing upon said control means a negative bias potential-of such magnitude as to substantially suppress the flow of current in all portions of said path, and adjustable means for reducing the bias potential 'applied to said control'rneans, thereby to bring into operation and to control the current flow in the said different portions of said path without modulation distortion of said desired signals or production of cross-talk effects between said desired signals and undesired strong signals.

3. A method of amplifying radio frequency,

signals from a desired signal source inthe strong signal field of an undesired signal lsource which consists in producing two electron streams owing at such different rates in parallel in the amplifier that the amplifier has a continuously variable mu factor, impressing said desired signals upon the amplifier input whereby the signals are magnified in intensity in the amplifier output through said electron streams, and regulating thev ow of said streams between extremely wide limits of negative bias potential to control the signal intensity in the amplifier output without modulation distortion of the desired signals or production of cross-talk eiects between the said desired signals and said undesired signals.

4. A method ofamplifying radio frequency signals of variable intensity from a desired signal source in the strong signal field of an undesired signal source which consists in producing two electron streams flowing at such dierent rates in parallel in the amplifier that the amphiier has a continuously variable mu factor, impressing said desired signals upon theV amplifier input whereby the signals 'are magnified in intensity in the amplifier output through said electron streams, and regulating the ow of said streams between extremely wide limits of negative bias potential to maintain the signal intensity in the ampliier output substantially constant without modulation distortion of the desired signals or production of cross-talk effects between the said desired signals and said undesired signals.

5. In a radio frequency amplifier, cathode and anode means, an energizing circuit for said cathode means and means for maintaining the anode means at a positive potential with respect to the cathode means so as to establish'a flow of space current between said cathode and anode means,

control means in the path of space current flow,

an input circuit including a source of biasing potential for the control means, said input circuit being connected between the cathode means and Ithe control means for impressing desired radio frequency signals on said amplifier, an output circuit connected between the cathode means and the anode means, said control means being arranged so as to exercise'substantially different degrees of'control over space 'current flow along vdifferent portions of the path of space current flow as the bias potential on the control means is varied, said source of biasing potential ybeing arranged so as to impress upon the control means a negative bias potential of such magnitude as to suppress the flow of current in all portions of said path and adjustableV means for progressively reducing the bias potential applied to said control-means thereby to bring successively into operation and t'o control the current flow in the said different portions of said path without modulation distortion of the 'desired signals or production of cross-talk eects between the desired signals and undesired signals.

6. A method of amplifying certain desired radio frequency signals fed from a signal source to a signal amplifier in the presence of certain other undesired radio frequency signals without causing modulation distortion of the vdesired signals or the production of cross-talk eects between the said desired signals and the other signals which includes the steps of producting at least two electron streams flowing at such different rates in parallel in the amplifier that the amplifier is caused to have a continuously variable mu factor, impressing thev desired signals upon` the amplier input whereby the signals are magnied in intensity in the amplifier output through the action of the electron streams and simultaneously regulating the flow of said streams at different rates but by means of identical variations of negative grid bias potential to control the signal intensity in the amplifier output.

7. In a radio frequency amplifier, a source of oscillations to be amplified and a utilizing circuit for the amplified oscillations, a coupling between the source and the utilizing circuit comprising at least two parallelly arranged electronic devices each thereof being provided with an anode, a cathode and at least one grid electrode, a common input circuit for said devices and means for connecting the input circuit to said source, a common output circuit for said devices and means for connecting' the output circuit to the utilizing circuit, one of said electronic devices having a comparatively low amplification factor said other device having a higher amplification factor and a common means for varying the amplification factors of both devices simultaneously.

8. In an electrical circuit,a source of oscillations and a utilizing circuit, a coupling means beelectronic-relay. including a high mu tube and a low mu tube, -a common input circuit for both tubes, circuit means tor connecting the source to the common input circuit and arranged so as t0 impress oscillations from the source, upon both tubes simultaneously and in like phase. a common output circuit for both tubes, the space path of each of said tubes being in parallel relationship with respect to the common output circuit,

means for connecting the 'common output circuit to the utilizing circuit,'said two tubes having a combined plate current-grid voltage characteristic which is in effect the characteristic curve of a high mu tube the curvature of which in the vicinity of the lower knee thereof, is reduced to such an extent that the cut-o8 point of the curve is considerably extended toward the negative grid potential region.

10. In an electrical circuit, a source of signal` ode and control means being so formed and ar-` ranged with respect to each other that the flow ot space current between the anode means and the cathode means is through two paths, one path having high mu characteristics and the other path having low mu characteristics, means for varying the bias potential applied to the control means to thereby vary the flow oi' space current between the anode means and the cathode means. like increment changes in said bias potential exercising dinerent degrees oi' control over the two paths, an input circuit for the relay means, means for electrically connecting the input circuit to the source whereby signal voltage is transferred from the source to the relay means, an output circuit connected betweenthe anode means and the cathode means, said output circuit including a source of space current. and means for electrically connecting the output circuit to the utilizing circuit.-

' l sones 1lr In a signalling circuit, a source d signal modulated carrier voltage and'a utilizing device, means establishing a multi-mu space current path between the source andthe utilizing device for transferring signal voltage effects from the source to the utilizing device, grid means interposed in said spacepath, means for applying` a potential to the `grid means, said space current path having a space current-'grid voltage characteristic which is in eifect the characteristic curve of a high mu electronic tube the curvature of which is reduced, in the vicinity of the lower knee thereof, to such an extent that the cut-oil' point of the curve is considerably extended into the negative grid potential region, and means for varying the potential applied to the grid means' to control the flow of space current in said space current path, variations in said potential exercising different degrees of control along different sections of the space current path, whereby Athe degree of transfer of, signal voltage effects from the source to the utilizing device may be varied over wide limits without modulation distortion of the desired signal voltage or production of cross talk effects. l2. In a signalling circuit, an iiput circuit for signal modulated carrier voltage and a utilizing.

device, means comprising cathode, -grid and an- `ode means for establishing between said input circuit and said utilizing device to transfer signal voltageeffects from the input circuit to the utilizing device. a multi-mu space current path in which the'grid means at any operating bias po-4 tential exercises different degrees of control along dii'i'erentl sections of the space current path, o which the space current-grid voltage characteristic is in eii'ect the characteristic curve of ,a high mu tube with its curvature in the vicinity of the lower knee extending far into thefnegative grid bias region and resembling substantially the char-v acteristic curve of a low mu tube, and means i'or varying the bias potential of said grid means to control the flow of space current in the space current path. STUART BALLANTINE. 

